21518921 has 4 divisors (see below), whose sum is σ = 21630612. Its totient is φ = 21407232.

The previous prime is 21518881. The next prime is 21518933. The reversal of 21518921 is 12981512.

It is a semiprime because it is the product of two primes.

It can be written as a sum of positive squares in 2 ways, for example, as 13424896 + 8094025 = 3664^2 + 2845^2 .

It is a cyclic number.

It is not a de Polignac number, because 21518921 - 2⁶ = 21518857 is a prime.

It is a super-2 number, since 2×21518921² = 926127922008482, which contains 22 as substring.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (21518971) by changing a digit.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 55556 + ... + 55941.

It is an arithmetic number, because the mean of its divisors is an integer number (5407653).

Almost surely, 2^21518921 is an apocalyptic number.

It is an amenable number.

21518921 is a deficient number, since it is larger than the sum of its proper divisors (111691).

21518921 is a wasteful number, since it uses less digits than its factorization.

21518921 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 111690.

The product of its digits is 1440, while the sum is 29.

The square root of 21518921 is about 4638.8491029565. The cubic root of 21518921 is about 278.1464344209.

The spelling of 21518921 in words is "twenty-one million, five hundred eighteen thousand, nine hundred twenty-one".